Thyroid hormones play an important role in the modulation of energy metabolism by regulating the rate of thermogenesis, i.e. the amount of heat produced to maintain the stable core temperature of the organism. Clinically, hypo- and hyperthyroidism are associated with major changes of thermoregulation and energy expenditure. Nevertheless, the ability of thyroid hormones in regulating the temperature and energy expenditure in response to changes of temperature is poorly understood in humans. In a collaborative randomized, cross-over study, we are now analyzing the changes in the thyroid hormones, energy expenditure and stress hormones in response to exposure to mild changes in environmental temperature using a metabolic chamber. Changes in skin and core temperature, heart rate, energy expenditure, circulating thyroid hormones, ACTH, cortisol, catecholamines, free fatty acids and glucose are being monitored. The results gathered from this study are expected to provide a comprehensive insight into the mechanism(s) regulating the energy metabolism in response to mild cold. They might lead to the characterization of a population of subjects who are requiring excessive energy expenditure to prevent hypothermia upon cold exposure.